Number or ratio expressed as a portion of 100
A percentage is a number or ratio expressed as a fraction of 100. It is often denoted using the percent sign, "%".
The word "percentage" goes back to the latin words "centum" which mean 100 in English. "Per centum" can be literally translated as "from hundred".
Often percentages are depicted as pie charts summing up to 1, i.e. 100 %. Often a percentage is also used to show a relative change, e.g. in 2013 the GDP growth rate in the USA was 2.2 percent which mean that:
GDP USA 2013 = 102.2 GDP USA 2012